The present invention relates to an alignment guide. In particular, the present invention relates to an alignment guide for aligning an instrument, more particularly a surgical instrument.
During surgical procedures it is commonly necessary to accurately locate prostheses and instruments. For instance, during implantation of an acetabular cup into a pelvis, it is important to ensure that the cup is accurately located within a reamed cavity. Typically, such a procedure requires a number of separate instruments to be used. It is important to ensure that the alignment determined for the cup during initial surgical steps is maintained during later surgical steps. For instance, position of the cup may be initially determined. The cavity must then be reamed. It is important that the desired version angle and inclination (that is, the varus-valgus angle) of the cup is maintained during the reaming by ensuring that the reamer is aligned correctly relative to the instruments used to determine the initial position of the cup to ensure that the cavity is correctly located. Additionally, it is important to either detect that the patient's pelvis has moved during the surgical procedure, or to ensure that the alignment of the acetabular cup has also shifted by a corresponding amount.
Inclination and version guides are widely used during total hip arthroplasty to assist in aligning the acetabular cup. Separate guides may be provided for inclination and version angle. Alternatively, a combined guide may be provided. One known form of combined alignment guide enables the position of the acetabular cup to be set at an angle relative to the floor of the operating room and the long axis of the patient. However, such an alignment takes no account of movement of the patient on the operating table during surgery. Regardless of changes in the patient's position, the alignment guide indicates the same orientation of the acetabular cup. Consequently, either the patient must be maintained in a strictly lateral position or the surgeon must be alert to spotting changes in the desired cup position caused by the patient moving.
An alternative acetabular cup alignment guide, which is commercially available from San-Tech Surgical Sail and described in PCT publication WO-2005/009303, uses a pair of spirit levels. A first spirit level is coupled to the patient's pelvis to indicate if the patient's position has changed. A second spirit level is coupled to the surgical instrument. Changes in inclination of the instrument can be detected by a shift within the second spirit level. Changes in the version angle are detected by referring to the relative position of the first spirit level and a wire arm which extends from the second spirit level. Whilst offering improved accuracy, such an alignment guide is relative difficult to use as it comprises multiple separate components and requires multiple measurement steps.